High resolution absolute encoder

ABSTRACT

A high resolution encoder device using a number of static sensors distributed on a circumference, and a rotating disc, having several sections of two different properties on an annular track according to a predefined pattern, placed so that the sensors can sense the properties of the sections of track in proximity. An auxiliary unit is also provided and by itself or in combination with the sensor signal values, provides a first low resolution position value. In a first processing step, the signals of each sensor are compared to a threshold, and bit values zero or one for each sensor are set according to the comparison result. All bits are then set in a digital word, in order to create a code number, which in combination with the output of the auxiliary unit, is characteristic of a first low resolution position value. For each low resolution position value, a mathematical combination of signals values is defined. The values of the result of said mathematical combination of the signals value is then used as an entry variable to pre-recorded tables to output high resolution position value.

FIELD OF THE INVENTION

The present invention relates to encoder devices (encoders).

BACKGROUND OF THE INVENTION

Encoders are used to measure the angular position of a rotating element,or the relative displacement of sliding elements; they are typicallyused in control systems, often referred as servo systems, where a motioncontroller is used to make a moving element to follow a precise desiredpath. For that purpose, encoder devices include an electronic interfacewhich allows their connection to a motion controller.

An encoder may be of two types, rotary and linear. Rotary encoders aredesigned to measure the angular position of a rotary element, like theshaft of a motor or any rotating device. Linear encoders are designed tomeasure the relative movement of two sliding elements, for example asliding carriage mounted on linear bearing relative to a static base.

In a common application, a rotary encoder is mounted on an electricalmotor shaft at the rear end, and provides position information about theshaft rotation angle to the electric motor controller and this at a highrate. The motor controller will then output a current to the motor inorder to make it rotate toward the desired position.

In another common application, a linear encoder is mounted on the movingelement of a linear motor, and is connected to the linear motor motioncontroller.

In the scope of this patent application, the term “encoder device” isused to refer either to a rotary or to a linear encoder. A linearencoder is constructed by using the same components as a rotary encoder,and disposing these components on a linear path. The linear path isdivided into a number of consecutive equal length section referred toherein as periods. In a first implementation, encoder components aredisposed along one period in a similar way to the disposition of thesecomponents on a circular path. In other implementations, the componentscan be disposed on several periods, provided that the distance of eachcomponent from the position of the same component in the firstimplementation is equal to an integer number of periods.

In automatic machinery, it is often required that moving elements willfollow a path with a very high precision, and at high speed. To achievethis, the encoder device must be designed with high precision, andshould be able to transfer position information at a high rate. As anexample, commercially available rotary encoders can provide precisionbetter than 0.01 degree; rate of transfer of the rotation angle value toa motion controller is typically between 8,000 to 30,000 value transfersper second.

Another parameter for designing an encoder device is its resolution. Theresolution represents the smallest position change that the encoderdevice is able to measure in one turn or in one length unit, and isusually expressed in number of position values per turn or lineardistance. The smallest position change defined by the resolution isusually smaller than the precision, meaning that the encoder device isable to provide position value having more significant digits thanrequired for the precision, even if the position value output differsfrom the actual position by some error, this error being inferior tothat defined by the precision characteristic of the encoder. Highresolution allows motion controllers, also called servo controllers, toachieve a tight and smooth control of the moving elements.

Encoder devices may be absolute or incremental. An absolute encoderdevice is able to measure the angular or linear position relative to afixed reference position, while an incremental encoder device is onlycapable of measuring the angular or linear displacement from the startof its operation. Thus, when an incremental encoder device is used inautomatic machinery, it is of common use to execute, at each start ofoperation of the machine, a search for a reference position. This searchis done at slow speed in a given direction, until a limit switch orother device placed at the reference position is activated. This searchprocedure adds complexity to the system, and delays the first operationof the machine. In spite of this drawback, incremental encoders arecommonly used, due to their simplicity and their low cost. In manycases, a machine builder would have preferred to use an absoluteencoder, but makes use of an incremental encoder due to the higher costof presently available absolute encoders.

It is desirable to provide an absolute encoder device, which is ofsimple fabrication and provide high precision and resolution at a lowcost.

In Patent U.S. Pat. No. 9,007,057 by Villaret, there is described anabsolute encoder device of simple construction, that can provideabsolute position information with high resolution. The device makes useof a number n of analog sensors, equally distributed on a circumference,on a static part; a rotating disc, having sections of alternatingproperties on an annular track according to a specific pattern, isplaced so that the sensors can sense the property of the section oftrack in proximity. During disc rotation, different sections of therotating disc get in proximity to each sensor. Each sensor electricalsignal is first digitized to provide a bit value 1 or 0; Bit values ofall sensors are then combined in a digital word to create a unique codevalue for each rotating disc angular range position. In a second step,one of the n sensors is selected and its analog output value is used tocalculate a high resolution position value.

An advantage of the above-mentioned patent is the simplicity of thedevice. Since sensors are equally distributed on a circular line, thedistance between sensors is relatively large and commercially availablesensors of normal size can be used.

The term “sectors” is defined in the above-mentioned patent as beingangular portions of an encoder rotating disc circular track, all sectorsbeing of approximately equal angular size. Each sector of the said trackis made of material having a first or a second property, according to apredefined pattern.

A first requirement of the above-mentioned patent is that the code ismonovalent, i.e. that a code value is obtained only on the range of onesector.

A second requirement is that the code should be a Gray code, i.e. duringmovement, the transition from one sector to the neighbor sector willresult in the change of only one bit of the digital code. This isrequired in order to avoid code errors during transition.

Both above requirements can be obtained using a pattern designedaccording to method described in patent U.S. Pat. No. 8,492,704 byVillaret.

These two requirements result in practical implementation limitations,as explained below:

Overall achievable resolution of the encoder according to U.S. Pat. No.9,007,057 is roughly equal to the resolution of the digital codemultiplied by the resolution of the analog sensors signals.

For the purpose of increasing resolution, either a) the resolution ofthe analog sensor reading orb) the resolution of the digital code shouldbe increased.

Regarding a), practical resolution of standard Analog sensors and Analogto digital converters is limited by the electrical noise. In particular,a high resolution encoder should also provide the position value withina very short time, thus requiring a high speed Analog to digitalconverter. Increasing resolution thus becomes very difficult andimpractical.

Regarding b), increasing resolution of the digital code can be done byincreasing the number of sensors and changing the pattern according topatent U.S. Pat. No. 8,492,704 by Villaret. However, when increasing thenumber of sensors n, the code resolution obtained becomes veryinefficient, i.e. the number of codes becomes a smaller fraction of thenumber of possible code values 2{circumflex over ( )}n. For example,with n=7 sensors, it is possible to find practical patterns providing 98code values. Using n=8 sensors, a practical pattern can provide only 128code values. So while the number of sensors increases, gain inresolution is small. In another aspect, for the example of n=7 sensors,a practical pattern can be found that defines sections extending over arelative large number of sectors, so that they can be easily implementedwith elements of relative large size, for example with magnets. For ahigher number of sensors, sections of the pattern have much smaller sizeand become impractical for implementation, requiring miniaturecomponents, and for example become impractical for magnets.

For the purpose of further increasing the resolution, it is thus desiredto provide an encoder device providing a higher resolution of thedigital code and a higher resolution of the analog processing of thesensor signals.

Thus it is an object of the present invention to provide an encoderdevice able to provide high resolution including a) an auxiliary unit toprovide a first low resolution position value whose resolution can begreater than 2{circumflex over ( )}n, where n is the number of sensorsand b) a pattern on a moving element and a number n of static sensors tosense this pattern and c) a processing unit and method to increase theprecision of the encoder. The first low resolution position value isobtained by combining the output of the auxiliary unit with a digitalcode calculated from the sensors analog signals.

A first advantage of this invention is that the resolution of this firstposition value is not limited to 2{circumflex over ( )}n, where n is thenumber of sensors. Since the total resolution of the encoder positionoutput is the product of the resolution of the low resolution positionvalue with the resolution of the analog signals, then the totalresolution of the encoder is increased.

A second advantage is that electrical signals of sensors aremathematically combined to provide combined signals that are lesssensitive to tolerances in the mechanical and electronic components.

It is also an object of the present invention to provide the abovementioned high resolution encoder device without the above mentionedauxiliary unit, wherein the above mentioned first low resolutionposition value is provided with a high resolution by processing of theanalog sensors signals, and wherein this low resolution can be higherthan 2{circumflex over ( )}n where n is the number of sensors.

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present invention toprovide a high resolution encoder device having sensors preferablyequally distributed on a circular line, so that commercially availablesensors can be used. The new encoder device makes use of sensorsproviding an analog output and includes a memory and processing means inorder to obtain a high absolute resolution, not limited by the number ofsensors.

In a first aspect, there is provided a low resolution position valuewith higher resolution than 2{circumflex over ( )}n, where n is thenumber sensors, thus allowing a total higher resolution of the encoder.In a second aspect, the analog processing of the electrical signalsincreases the precision of the encoder output.

The present invention makes use of a number of static sensors preferablyequally distributed on a circumference; a rotating disc, having severalsections of two different properties on an annular track according to apredefined pattern, is placed so that the sensors can sense theproperties of the sections of track in proximity. During disc rotation,different sections of the rotating disc are positioned in proximity toeach sensor. According to this invention, the sensor signals take analogvalues continuously varying from a first range of values when inproximity to a rotating disc section of a first property, a second rangeof values when in proximity to a rotating disc section of a secondproperty, and intermediate values when in proximity to a transitionposition on the rotating disc.

The present invention also includes an auxiliary unit. This unit, byitself or in combination with the sensor signal values, provides a lowresolution position value. Any known prior art technology can be used toprovide this low resolution position value.

Previous to first operation of the encoder, in a pre-processing step,characteristics of the encoder are calculated and measured, and storedin the encoder memory. These characteristics are in the form of tablesof values and predefined codes that define the response of the sensorsto the rotating disc track properties.

In a first type of embodiment, in a first processing step, the signalsof each sensor are compared to a threshold, this threshold being forexample the median value between the maximum and minimum values of thesignals, and bit values zero or one for each sensor are set according tothe comparison result. All bits are then combined in a digital word, inorder to create a code number. In the scope of the present invention, a“sector” is defined as a continuous range of positions for which a givencode number is obtained. The code number in itself does not provide amonovalent code, meaning that the same code may be obtained at differentsectors. However this code number, in combination with the output of theauxiliary unit, is characteristic of the sector. A sector is thusidentified by its code number and the output of the auxiliary unit.

In the above mentioned pre-processing step, the sector code and positionhave been pre-stored in memory, together with the output of theauxiliary unit. Identification of the sector then provides positioninformation with the resolution of the sector size. This position valueis further referred to herein as low resolution position value. Forexample, it will be shown that a pattern with 5 sensors can provides 50sectors. Then the low resolution position value can provide positioninformation with an approximate resolution of 1 turn/50 i.e. 7.2degrees.

In a second type of embodiment, the auxiliary unit is a low resolutionabsolute encoder of any prior art and the low resolution position valueis, for example, a rounded value of this encoder position output value.In these embodiments, a sector is thus defined as the range of positionsfor which the same low resolution position value is obtained.

In a third type of embodiment, an auxiliary unit is not used, and thelow resolution position value is obtained using the digital code andfurther processing of the analog sensor signals.

In a second processing step of any of these embodiments, the lowresolution position value is used to select a particular mathematicalcombination of a number of signals. The selection of the particularmathematical combination of signals for each low resolution positionvalue have been pre-defined in the pre-processing step mentioned above.The combined signal, calculated using the particular mathematicalcombination defined for this low resolution position value, provides asignal with reduced relative noise and less sensible to mechanicaltolerances, temperature and other factors affecting the precision of theencoder. The values of the combined signals are then used as entries topre-recorded tables to output high resolution position value.Characteristic of this invention is that the combination of signals ispre-defined per each low resolution position value.

The encoder is thus able to output a high resolution position value,implementing the following processing steps:

1) Reading a first low resolution position value using either a)auxiliary unit, or b) both auxiliary unit and digital code, or c)digital code and further processing of the analog signals;

2) Selecting, according to the low resolution position value:

-   -   a) the type of mathematical combination of signals to be used        using pre-stored combination tables and the low resolution        position value as an entry variable, and    -   b) the high resolution position table to be used, using a        pre-stored table of the high resolution position tables, the        entry variable to this table being the low resolution position        value;

3) Calculating combined signal(s) using the selected combination;

4) Using the combined signal(s) as entry(s) to the selected positiontables to obtain high resolution position value(s); and

5) Providing output position value(s), or a combination of them in caseof plural position values.

A characteristic of this invention is that the combination of signals ispre-defined per each low resolution position value.

For each low resolution position value, at least one analog value iscalculated as a mathematical combination of the sensor signals. For eachlow resolution position value, predefined combinations are defined thatcan include multiplications, additions or any other mathematicaloperations between signals. These values are further referred to hereinas combined signals. For each low resolution position, during thepre-processing step, a number of combined signals is associated.

In a first consideration, combination of several signals into one signalstatistically reduces the signal to noise ratio.

In a second consideration, by appropriate definition of the mathematicalcombination, the combined signal can be made less sensitive to theinfluence of physical factors like variation of signal amplitude withtemperature, or variations of the signal amplitude to movements of thedisc center of rotation axis.

For example, a small movement of the disc center of rotation axis willresult in a small eccentricity of the disc relative to sensors. Somesensors will get closer to the disc track, some other will get furtherfrom the disc track, and the sensors signal amplitude will be affectedaccordingly. If one sensor signal is used in the encoder processingalgorithm, as described in U.S. Pat. No. 9,007,057 by Villaret, then theencoder position output precision will be directly affected by thissensor electrical signal amplitude change. However, if several sensorsare used in the processing algorithm, each sensor is influenced in adifferent direction, and the total influence on encoder output isaveraged, resulting in increased precision.

In the scope of this patent, the range of positions in which the samecode, or the same low resolution position value is obtained is designedas a sector.

The particular combination of signals associated with each code ispredefined according to the characteristic of the signals depending onthe particular mechanical implementation and the type of sensor, withthe purpose to produce a monotonous function of the shaft angle withinthe sector.

During the pre-processing step there is also recorded as a function ofthe rotating disc position, for each sector, the functions (combinedsignal functions) expressing the variations within the range ofpositions of the sector of the combined signals. Encoder sensors androtating disc section properties are designed so that the transitionfrom a first range of values to a second range of values of the signalsextends over a range of rotating disc position equal to or larger thanone sector. Accordingly, within the range of one sector, it is alwayspossible to define a combination of signals which is monotonous with theangular disc position. Three examples of combinations are listed here,however many others are possible:

The weighted sum of all the signals that are monotonous within thesector, using positive weight if signal is growing, negative weight ifsignal is decreasing

The arc-tangent of the ratio of two signals that are zero crossing atthe two ends of the sector

The arc-tangent of the ratio of two combined signals each combinedsignal being the weighted sum of monotonous signals in the sector.

It must be understood that many other combinations can be used.

The combined signal values are recorded, during the pre-processing step,for each position in the respective sector and stored in tables ofvalues at high resolution, calculated by theory or simulation, ormeasured using a reference encoder. Thus for each sector, at least onetable is defined. Optionally, a mathematical model that approximates thevariations of the combined signal values with position may be found. Inthis case these tables will contain the parameters of the mathematicalmodel.

In a last processing step, combined signals associated with the sectorare selected, and their values are used as entries to the tables definedfor this sector. These tables have been recorded in the pre-processingstep with high resolution. There may be several tables defined for onesector; in that case several position values are obtained. These valuesmay slightly differ, depending on the precision of the tables and thesensor noise. Averaging or other methods can be used to calculate aposition value with optimal precision using these plural output values.

The result is thus a high resolution absolute encoder, of simplearrangement, that can provide improved resolution by use of an auxiliaryunit and combinations of signals, used as entries to pre-selectedposition tables.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an absolute encoder with sensors and auxiliary unitaccording to the present invention;

FIG. 2 shows an implementation of the encoder with a pattern of 5magnets, 5 sensors and two digital Hall sensors;

FIG. 3 shows the variations of the sensors signals for theimplementation shown in FIG. 2;

FIG. 4 shows an example of code values obtained with a pattern and 5sensors where same code value 25, shown as an example, is obtained attwo different sectors;

FIG. 5 shows a combined signal at a given angular position;

FIG. 6 shows a general block diagram for the processing of the sensorsignals to output a high resolution position value;

FIG. 7 shows sensor signal variations in the range of two sectors forwhich the same code is obtained; and

FIG. 8 is a block diagram describing the processing steps for obtaininga low resolution position value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 there is shown an encoder arrangement, according to thepresent invention. An encoder disc 101, fixed to a rotating shaft 103,includes at its outer circumference, a track with a number of sectorslike 102 a and 102 b of different properties. Sectors like 102 a filledin black color represent sectors of a first property, and sectors like102 b filled in white color represent sectors of a second property.Sensors 105 a, 105 b, 105 c, 105 d and 105 e are static and placed inproximity of the track. The sensors provide output analog signalsrespectively A0, A1, A2, A3, A4 and A5. The output of the sensor variesfrom a first range of values when the sensor is in proximity to a sectorof a first property like 102 a, and a second range of values when thesensor is in proximity to a sector of a second property like 102 b. Therotating encoder disc also includes a second track 110.

A static auxiliary unit 109, placed in proximity to the second track110, is connected to a microcontroller (CPU) 107, and provides a lowresolution position value available to the CPU 107. The second track 110and the auxiliary unit 109 may be designed according to any prior artmethod. Optimally, the auxiliary unit 109 and the second track 110 willbe designed to provide very low resolution for low cost considerations.The analog signals A0-A5 are input to an analog to digital converter 106and their digital values are then input to the CPU 107. The CPU 107 alsoreceives low resolution position value from the auxiliary unit. The CPU107 then processes the signals values A0-A5 and the low resolutionposition value according the method of this invention described furtherherein, and outputs a high resolution position value.

FIG. 2 shows an example of pattern of 50 sectors that can be used with 5sensors. The encoder disc includes an external cylindrical wall 200.Internally to this wall, 5 magnets having an arc shape 201 a to 201 eare affixed to form the pattern. The magnets are all magnetized radiallywith the same direction, for example inward. A pattern is obtainedwherein sections of the internal circumference of the cylindrical wall200 are covered with magnets (first property) or not covered withmagnets (second property). According to the specific pattern shown inFIG. 2, the magnets 201 a-201 e cover respectively and approximately 6,4, 4, 3 and 8 sectors, and the uncovered sections of the wall 200circumference adjacent to magnets 201 a-201 e in the counter clock wisedirection extend respectively and approximately over 3, 4, 4, 6 and 8sectors. Five static Hall sensors 202 a-202 e are uniformly positionedon a circular line, internal and concentric with the external wall 200and these sensors output a signal in relation with the magnetic field.Whenever there is a rotation of the disc, external wall 200 and magnets201 a-201 e rotate around the sensors.

During rotation, magnet-covered sectors and magnet-uncovered sectorsalternate and are positioned in proximity to the sensors, generating avariable sensor signal, which is a function of the disc rotation angle.Also shown in FIG. 2 is a circle magnet 204 fixed to the rotating discand magnetized so that a first half circle is North pole, and the otherhalf a South pole. Two static digital Hall sensors 203 a and 203 b areplaced close to the magnet, are connected to the CPU and can sense whichmagnet pole is in proximity. These two digital Hall sensors are placedat a 90 degree angular position distance. In this embodiment, thesedigital Hall sensors implement the above mentioned auxiliary unit andprovide position information with a ¼^(th) of a turn resolution to theCPU according to their two possible respective states i.e. in proximityto a North or South pole. The Boolean state of these digital Hallsensors will be further referred as H1 and H2.

FIG. 3 shows an example of the signals variations as a function of theangular rotational position of the encoder disc for the pattern of FIG.2. The five signals S1-S4 have basically the same shape with an angularshift according to the respective sensor angular position. Apart fromangular shift, signals S1-S4 may have small shape differences due totolerances in the geometrical arrangement or in the Hall sensorcharacteristics. We refer here to a zero cross of signal as the positionwhere the value of this signal crosses a threshold T0, typically closeto the average value. The pattern has been designed so that zerocrossings are approximately equidistant. At each position, the Booleanvalue 1 is associated with a signal if its value is greater than thethreshold T0, and zero if smaller. Thus, five associated Boolean valuesare set in a five bit word, thus creating a code for this position. Thenumber of sectors is equal to the number of zero crossings, and for thespecific pattern of FIG. 2 there are 50 sectors. Since the number ofcodes is limited by the number of sensors (5) to 2{circumflex over( )}5=32, then several sectors may have the same code.

FIG. 4 is a graph showing the code value for each sector. In FIG. 4there is also shown 4 ranges of positions for which the digital Hallsensors have a given state. These ranges are shown by double arrowslabeled with the state of the digital Hall sensors H1 and H2. Thecombination of the code of a sector, and the state of the digital Hallsensors provides a unique corresponding identifier of the sector withthe resolution of a sector size. As an example, in FIG. 4, it can beseen that the same code 25 is obtained for sectors A and B which aresectors 9^(th) and 33^(rd) when counting sectors from angle 0 of thegraph. At sector A, digital Hall sensors have states H1=0 and H2=0,while at sector B digital hall sensors have different states H1=1 andH2=1. The code of a sector combined with the digital Hall sensor's statethus gives a unique corresponding identifier of the sector, and thusgives low resolution position value of the disc rotation angle.

It is of particular advantage that this low resolution position value isobtained with a resolution greater than 2{circumflex over ( )}n, (32 inthis particular case). This increased resolution is obtained requiringneither an increased number of sensors nor miniaturization of thepattern sections.

This is in contrast with U.S. Pat. No. 9,007,057 by Villaret; in thisprior art patent, in order to obtain a high resolution, the number ofsensors must be increased. This also requires the use of small patternsections, precisely positioned. According to this prior art patent, thepattern section should have position and length precision better thanone half a sector size in order to produce the Gray code. In total, thenumber of sensors is increased and precision requirements of the patternare increased, resulting in higher complexity and cost. In particular,if magnets are used for pattern, increasing resolution of the digitalcode becomes impractical.

In a pre-processing step, for each sector, a least one particularcombination of a number of Hall sensor signals is selected and stored inthe form of tables in the CPU 107 memory. These tables are referred toas “combination tables”. A combination of Hall sensor signals mayinclude any mathematical operation. The purpose of the combination is toobtain a combined signal which is monotonous with the angular positionin the range of the sector. A further purpose of this combined signal isto create a signal which is less sensitive to mechanical and electronictolerance of the particular implementation. Examples of signalscombinations are:

A weighted sum of all the signals that are monotonous within theparticular sector. For example, weight may be 1 if the signal isgrowing, or −1 if the signal is decreasing

The arc-tangent of the ratio of two signals that are zero crossing atthe two sector ends

The arc-tangent of the ratio of two combined signals each combinedsignal being the weighted sum of monotonous signals in the code range.

It must be understood that many other combinations can be used.

Using combination tables, for each sector, identified as explainedabove, a particular combination of signals is defined. The CPU 107acquires the low resolution position value and uses it as an entryvariable to the combination tables, and calculates the correspondentcombined signals.

FIG. 5 shows the variation of the signals S1-S5 (also of FIG. 3) withina short angular position range. At the particular position P, the CPU107 acquires the low resolution position value, and selects thecorrespondent combination of signals to be used. In the particularexample shown in FIG. 5, the combination is defined by C=S1−S3. Thevariation of the combined signal C is shown.

During the pre-processing step, for each low resolution position valueand in the respective low resolution position range, combined signalsvalues and angular position are stored in position tables, as a functionof the combined signal values. Thus for each low resolution position, atleast one table is defined. Optionally, a mathematical model thatapproximates the variations of the combined signal values with positionmay be found. In this case, these tables will contain the parameters ofthe mathematical model.

Referring again to the particular case shown in FIG. 5, at position P,the particular position table is defined. This table defines a functionof the position P=f(C) , where C represents the value of the combinedsignal, and P the position value. The sector with code 25 is definedbetween the two zero crossing of signals S3 and S1, occurringrespectively at positions P1 and P2. As can be seen, the combined signalC is monotonous over the whole range of the sector. The function canthus be inversed. This inversed function P=f(C) has been recorded in thepre-processing step in a specific position table of high resolution.Alternatively, the signals S3 and S1 could also have been used, since inthis sector they are monotonous. At position P there is shown in FIG. 5the slope of the signals C, S3 and S1 with respective double arrowslabeled DC, D3 and D1.

As can be seen, the slope of signals S3 and S1 (D3 and D1) are muchsmaller than the slope of the combined signal. This means that a smallerror in reading of signal S3 or S1 would result in a relatively largeerror of the position reading P. In contrast, the slope of the combinedsignal is always higher, and thus same error in reading results in asmaller position reading error. For clarity purpose, the combined signalhas been shown using two signals. In a practical embodiment, a greaternumber of signals can be used with various mathematical combinations toreduce the sensitivity to errors in the analog signal value readings.

FIG. 6 is a simplified block diagram of the encoder processing method.

In first block 601, a position value of low resolution is acquired fromthe auxiliary unit. As an example, in the above-described embodiment,this is achieved by reading the state of the two digital Hall sensors(203 a and 203 b of FIG. 3),

In block 602, the output of the sensors is read providing analogsignals.

In block 603, the said analog signals are compared to a threshold toprovide digital values and combined to make a digital word code.

In block 604 the code value combined with the position value of lowresolution of the auxiliary unit provides a unique correspondingidentifier of a position value of intermediate resolution. The obtainedcode value may be ambiguous due to the fact that it is characteristic ofa number of positions, however the low resolution position valueacquired from the auxiliary unit resolves this ambiguity.

Other means of reading the position value of intermediate resolution maybe used.

In block 605, the position value of intermediate resolution is used asan entry variable to the tables:

A first table defines the type of mathematical combination to use withthe signals.

A second table defines which position table to use.

The defined combination and position tables are then selected.

In block 606 a combined signal is calculated.

In block 607, the position value of high resolution is read from theselected position table, using the combined signal value as an entryvariable to the selected position table.

The output of block 607 is thus a high resolution position value outputby the encoder.

This process is executed in a continuous cycle 608.

In the above described embodiment, an auxiliary unit is required inorder to resolve code ambiguities. The auxiliary unit is preferably ofvery low resolution in order to reduce its cost.

In order to avoid additional cost of this auxiliary unit, a preferredembodiment will be shown which is able to provide the positioninformation of intermediate resolution by means of further analog signalprocessing.

In FIG. 7 there are shown the variations of the signals S1-S5 of theembodiment shown in FIG. 2. The signals are the same signal as shown inFIG. 3, and are shown in two graphs around the positions of sectors Aand B of FIG. 4. At these two positions, the same digital code 25 isobtained. In the left graph (FIG. 7) around A, it can be seen thatsignal S4 always has a value greater than a threshold value T1, while inthe right graph around B, the signal S4 always has value smaller thanthreshold value T1. The two sectors can thus be distinguished bycomparing the value of the signal S4 to the threshold value T1. In thepre-processing step, the value of the threshold T1, and the number ofthe signal to be compared to this threshold are recorded and stored inthe CPU 107. In case there are several sectors with the same code, thenseveral thresholds and signals like T1 and S4 can be used. For example,for the particular case shown in FIG. 7, two thresholds T1 and T2 can beused respectively with signals S4 and S2.

Finally, it is thus possible to obtain the position value ofintermediate resolution by further processing of the analog signals.

FIG. 8 shows a block diagram for the processing of the analog signals inorder to obtain a position value of intermediate resolution.

In block 801, all analog signals values are acquired.

In block 802, all signal are compared to a threshold T0 to provide aBoolean value and calculate a digital code.

In block 803 the digital code is used as an entry variable to the tablesto select:

1—A number of threshold values T1,T2.

2—Signals to be compared to these thresholds (n1, n2 . . . )

3—Table of positions of intermediate resolution.

In block 804 binary values are associated to selected signals (n1, n2 .. . ) by comparing each to the correspondent threshold. These binaryvalues are set in additional bits of the code, thus creating a secondarycode.

In block 805, secondary code is used as entry to the selected table oflow resolution position, and low resolution position value is obtained.

Many variations can be conceived within the scope of this invention,wherein the use of several thresholds provides a position value ofintermediate resolution.

Having described the invention with regard to certain specificembodiments, it is to be understood that the description is not meant asa limitation, since further modifications will now become apparent tothose skilled in the art, and it is intended to cover such modificationsas fall within the scope of the appended claims.

1. A high resolution encoder device to measure the angular position of arotating element, comprising: a) first means to provide a first angularposition value; b) a rotating disc fixed to the rotating element, saidrotating disc including a circular track having sections associated witha first and second property according to a given pattern, c) a number offixed sensors positioned in proximity of said circular track, eachsensor outputting an electrical signal having a first range of analogvalues when in proximity to said circular track section associated withsaid first property, and a second range of analog values when inproximity to said circular track section associated with said secondproperty, each sensor receiving continuously changing intermediatevalues when said rotating disc rotates from a position at which saidsensor is in proximity to said circular track section associated withsaid first or second property to a position at which said sensor is inproximity to said circular track section associated with said second orfirst property respectively, d) processing means to process the analogvalues of said electrical signals; and e) a memory to store pre-recordedtables of signal selection, signal combinations and table positionvalues, wherein for every said first angular position value there isselected: i) a number of said electrical signals, ii) at least onemathematical combination of analog values of said selected electricalsignals and iii) at least one of said pre-recorded table positionvalues, wherein said selected electrical signal analog values arecombined using said selected mathematical combination to provide acombined value, and wherein at least one high resolution angularposition value of said rotating disc is retrieved from at least one ofsaid selected table position values using said combined analog value asan entry variable.
 2. The encoder device of claim 1 wherein said firstmeans to provide a first position value comprises an auxiliary unitproviding a low resolution position value and wherein said electricalsignal analog values are digitized by comparison to threshold values toprovide a digital word code, and wherein said code and low resolutionposition value taken together are characteristic of said first positionvalue.
 3. The encoder device of claim 1 wherein said first means toprovide a first position value includes processing steps of: a)associating a Boolean value to each electrical signal by comparing it toa first threshold to compose a digital word code; b) selecting for saiddigital word code at least one electric signal, at least one secondthreshold value and at least one table of said first position values; c)obtaining at least one Boolean value by comparing said selectedelectrical signal analog value to one of said selected second thresholdvalues; and d) retrieving said first position value from said selectedposition value table using said Boolean values as entry variable to saidselected table.
 4. The encoder device of claim 1 wherein at least onesection associated with said first or second properties includespermanent magnets generating a magnetic field, and wherein said sensorsoutput said electrical signal analog values related to said magneticfield.
 5. The encoder of claim 2 wherein said auxiliary unit includestwo magnetic sensors distanced angularly by approximately one quarter ofa turn and at least one magnet fixed to said rotating disc, said magnetinducing variable electrical signals in said magnetic sensors duringdisc rotation.
 6. A method for measuring the angular position of arotating element, using a high resolution encoder device comprising:providing a high resolution encoder device comprising: a) first means toprovide a first angular position value; b) a rotating disc fixed to therotating element, said rotating disc including a circular track havingsections associated with a first and second property according to agiven pattern, c) a number of fixed sensors positioned in proximity ofsaid circular track, each sensor outputting an electrical signal havinga first range of analog values when in proximity to said circular tracksection associated with said first property, and a second range ofanalog values when in proximity to said circular track sectionassociated with said second property, each sensor receiving continuouslychanging intermediate values when said rotating disc rotates from aposition at which said sensor is in proximity to said circular tracksection associated with said first or second property to a position atwhich said sensor is in proximity to said circular track sectionassociated with said second or first property respectively, d)processing means to process the analog values of said electricalsignals; and e) a memory to store pre-recorded tables of signalselection, signal combinations and table position values, and selecting,for every said first angular position value: i) a number of electricalsignals; ii) at least one mathematical combination of analog values ofsaid selected electrical signals and iii) at least one of saidpre-recorded table position values, wherein said selected electricalsignal analog values are combined using said selected mathematicalcombination to provide a combined value, and wherein at least one highresolution angular position value of said rotating disc is retrievedfrom at least one of said selected table position values using saidcombined value as an entry variable.
 7. The method of claim 6 whereinsaid first means to provide a first position value comprises anauxiliary unit providing a low resolution position value and whereinsaid electrical signal analog values are digitized by comparison tothreshold values to provide a digital word code, and wherein said codeand low resolution position value taken together are characteristic ofsaid first position value.
 8. The method of claim 6 wherein said firstmeans to provide a first position value performs the processing stepsof: a) associating a Boolean value to each electrical signal bycomparing it to a first threshold to compose a digital word code; b)selecting for said digital word code at least one electric signal, atleast one second threshold value and at least one table of said firstposition values; c) obtaining at least one Boolean value by comparingsaid selected electrical signal analog value to one of said selectedsecond threshold values; and d) retrieving said first position valuefrom said selected position value table using said Boolean values asentry variable to said selected table.
 9. The method of claim 6 whereinat least one section associated with said first or second propertiesincludes permanent magnets generating a magnetic field, and wherein saidsensors output said electrical signal analog values related to saidmagnetic field.
 10. The method of claim 7 wherein said auxiliary unitincludes two magnetic sensors distanced angularly by approximately onequarter of a turn and at least one magnet fixed to said rotating disc,said magnet inducing variable electrical signals in said magneticsensors during disc rotation.